Golf ball material and golf ball

ABSTRACT

A mixture having good thermal stability, flow and moldability and suitable as a golf ball-forming material is obtained when a base resin comprising a (metal ion-neutralized) olefin-unsaturated carboxylic acid-unsaturated carboxylate ternary random copolymer and optionally a (metal ion-neutralized) olefin-unsaturated carboxylic acid binary random copolymer is blended with specific proportions of a fatty acid and/or fatty acid derivative and a basic inorganic metal compound capable of neutralizing acidic groups left unneutralized in the base resin and fatty acid. Using the same material, high-rebound golf balls can be effectively manufactured.

The present invention relates to golf ball materials which have goodthermal stability, flow characteristics and moldability, and which arecapable of providing high-performance golf balls endowed withoutstanding rebound energy. The invention relates also to golf ballsmade with such golf ball materials.

BACKGROUND OF THE INVENTION

Over the past few years, wide use has been made of ionomer resins ingolf ball cover materials, also referred to hereinafter as “coverstock”. Ionomer resins are ionic copolymers composed of an olefin suchas ethylene in combination with an unsaturated carboxylic acid such asacrylic acid, methacrylic acid or maleic acid, wherein the acidic groupsare partially neutralized with metal ions such as sodium, lithium, zincor magnesium ions. They have excellent characteristics such asdurability, resilience and scuff resistance.

Ionomer resins account for most of the cover stock resin in current use.A variety of improvements have been made thereon because golfers arealways on the lookout for golf balls having a high rebound and excellentflight characteristics.

Related improvements taught by the prior art (see U.S. Pat. No.5,312,857, U.S. Pat. No. 5,306,760, and International Application WO98/46671) include cover stock in which a large amount of metallic soapis added to the ionomer resin to improve the cost and reboundcharacteristics of the ionomer cover stock.

These cover stocks, however, fall far short of practical levels. Duringinjection molding, the metallic soap in the cover stock decomposes andvaporizes to generate a large amount of fatty acid gas, often causingmolding defects. In addition, gas constituents settle on the surface ofthe molded part to considerably lower the paintability thereof.Moreover, although such cover stock in which a large amount of metallicsoap has been added to the ionomer resin does exhibit a rebound which isabout the same as or better than that of metallic soap-free ionomercover stock having the same degree of hardness, the improvement inrebound is not all that large. Indeed, depending on the type of metallicsoap used, the moldability and rebound of cover stock may in fact beseverely compromised.

SUMMARY OF THE INVENTION

Therefore, one object of the invention is to provide golf ball materialswhich have good thermal stability, flow and moldability, and which arecapable of producing golf balls with outstanding rebound. Another objectof the invention is to provide golf balls made using such golf ballmaterials.

Quite unexpectedly, it has been found that a mixture having good thermalstability, flow and moldability is obtained when 100 parts by weight ofa base resin having (a) an olefin-unsaturated carboxylic acid binaryrandom copolymer and/or a metal ion-neutralized olefin-unsaturatedcarboxylic acid binary random copolymer blended with (b) anolefin-unsaturated carboxylic acid-unsaturated carboxylate ternaryrandom copolymer and/or a metal ion-neutralized olefin-unsaturatedcarboxylic acid-unsaturated carboxylate ternary random copolymer in aweight ratio of 0:100 to 30:70 is blended with (c) 5 to 80 parts byweight of a fatty acid and/or fatty acid derivative having a molecularweight of 280 to 1,500 and (d) 0.1 to 10 parts by weight of a basicinorganic metal compound capable of neutralizing acidic groups leftunneutralized in the base resin and component (c). The mixture issuitable for molding and a molded part thereof exhibits improved reboundcharacteristics.

The molded part of the above-described golf ball material can form anydesired constituent component of a golf ball (which is selected fromamong a one-piece golf ball, solid core, solid center, cover and othercomponents).

The golf ball thus constructed has very good rebound and improvedinitial velocity performance. The invention is predicated on thesefindings.

According to the invention, there is provided a golf ball materialcomprising a mixture which is composed of essential components:

100 parts by weight of a base resin having (a) an olef-in-unsaturatedcarboxylic acid binary random copolymer and/or a metal ion-neutralizedolefin-unsaturated carboxylic acid binary random copolymer, blended with(b) an olefin-unsaturated carboxylic acid-unsaturated carboxylateternary random copolymer and/or a metal ion-neutralizedolefin-unsaturated carboxylic acid-unsaturated carboxylate ternaryrandom copolymer in a weight ratio of 0:100 to 30:70,

(c) 5 to 80 parts by weight of a fatty acid and/or fatty acid derivativehaving a molecular weight of 280 to 1,500; and

(d) 0.1 to 10 parts by weight of a basic inorganic metal compoundcapable of neutralizing acidic groups left unneutralized in the baseresin and component (c).

In a preferred embodiment, the mixture when molded has a Shore Dhardness of 30 to 60; the mixture has a melt index of 0.5 to 20 dg/min.;the metal ion-neutralized random copolymer in the base resin includes azinc ion-neutralized ionomer resin. The total content of randomcopolymers and the total content of metal ion-neutralized randomcopolymers in the base resin are preferably in a weight ratio of 0:100to 60:40. Component (c) is typically selected from among stearic acid,behenic acid, arachidic acid, lignoceric acid and derivatives thereof.Typically, component (d) is calcium hydroxide.

In another aspect, the invention provides a golf ball comprising amolded part of the golf ball material set forth above as a constituentcomponent.

DETAILED DESCRIPTION OF THE INVENTION

The golf ball material of the invention contains as an essentialcomponent a base resin having (a) an olefin-unsaturated carboxylic acidbinary random copolymer and/or a metal ion-neutralizedolefin-unsaturated carboxylic acid binary random copolymer blended with(b) an olefin-unsaturated carboxylic acid-unsaturated carboxylateternary random copolymer and/or a metal ion-neutralizedolefin-unsaturated carboxylic acid-unsaturated carboxylate ternaryrandom copolymer in a specific weight ratio.

In the base resin, the olefin, whether it belongs to component (a) or(b), generally has at least 2 carbon atoms, but not more than 8 carbonatoms, and preferably not more than 6 carbon atoms. Illustrativeexamples include ethylene, propylene, butene, pentene, hexene, hepteneand octene. Ethylene is especially preferred.

Suitable examples of the unsaturated carboxylic acid include acrylicacid, methacrylic acid, maleic acid and fumaric acid. Of these, acrylicacid and methacrylic acid are especially preferred.

The unsaturated carboxylate is preferably a lower alkyl ester of theforegoing unsaturated carboxylic acid. Illustrative examples includemethyl methacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butylacrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) isespecially preferred.

The olefin-unsaturated carboxylic acid binary random copolymer (a) orolefin-unsaturated carboxylic acid-unsaturated carboxylate ternaryrandom copolymer (b), to be commonly referred to as “random copolymer”hereinafter, may be prepared by furnishing the above ingredients andcarrying out random copolymerization in a well-known manner.

It is recommended that the unsaturated carboxylic acid content (simplyreferred to as acid content) within the random copolymer be adjusted toan appropriate level. For the random copolymer (a), an appropriate acidcontent is at least 4%, preferably at least 6%, more preferably at least8%, and most preferably at least 10% by weight, but not more than 30%,preferably not more than 20%, more preferably not more than 18%, andmost preferably not more than 15% by weight. For the random copolymer(b), an appropriate acid content is at least 4%, preferably at least 6%,and more preferably at least 8% by weight, but not more than 15%,preferably not more than 12%, and more preferably not more than 10% byweight. Outside the range, a lower acid content may lower resilience,whereas a higher acid content may is lower processability.

The metal ion-neutralized olefin-unsaturated carboxylic acid binaryrandom copolymer (a) or metal ion-neutralized olefin-unsaturatedcarboxylic acid-unsaturated carboxylate ternary random copolymer (b), tobe commonly referred to as “neutralized random copolymer” hereinafter,may be prepared by partially neutralizing acid groups in theabove-mentioned random copolymer with metal ions.

Examples of the metal ions for neutralizing acid groups include Na⁺, K⁺,Li⁺, Zn²⁺, Cu², Mg²⁺, Ca²⁺ Co²⁺, N²⁺and Pb²⁺. The use of ions such asNa⁺, Li⁺, Zn²⁺ and Mg²⁺ is preferred. Zn²⁺ is especially preferred.

Such neutralized random copolymers are prepared by neutralizing theabove-mentioned random copolymer with the foregoing metal ions. Forexample, the random copolymers can be neutralized using formates,acetates, nitrates, carbonates, hydrogencarbonates, oxides, hydroxidesor alkoxides of the metal ions. The degree of random copolymerneutralization with the metal ions is not critical.

Of the neutralized random copolymers, a zinc ion-neutralized ionomerresin is preferred because it allows the mixture to be increased in meltindex to an appropriate level to mold the material.

The base resin consisting of components (a) and (b) may be obtainedusing commercially available resins. For example, the random copolymer(a) is commercially available under the trade name of Nucrel 1560, 1214and 1035 from DuPont-Mitsui Polychemicals Co., Ltd. and ESCOR5200, 5100and 5000 from EXXONMOBIL Chemical. The random copolymer (b) iscommercially available under the trade name of Nucrel AN4311 and AN4318from DuPont-Mitsui Polychemicals Co., Ltd. and ESCOR ATX325, ATX320 andATX310 from EXXONMOBIL Chemical.

Also the neutralized random copolymer (a) is commercially availableunder the trade name of Himilan 1554, 1557, 1601, 1605, 1706 and AM7311from DuPont-Mitsui Polychemicals Co., Ltd., Surlyn 7930 from E. I.Dupont, and Iotek 3110 and 4200 from EXXONMOBIL Chemical. Theneutralized random copolymer (b) is commercially available under thetrade name of Himilan 1855, 1856, and AM7316 from DuPont-MitsuiPolychemicals Co., Ltd., Surlyn 6320, 8320, 9320 and 8120 from E. I.Dupont, and Iotek 7510 and 7520 from EXXONMOBIL Chemical.Zinc-neutralized ionomer resins such as Himilan 1706, 1855 and AM7316and Surlyn 9320 are especially preferred among the neutralized randomcopolymers.

The base resin is prepared by blending components (a) and (b) in aweight ratio between 0:100 and 30:70, preferably 0:100 and 20:80, morepreferably 0:100 and 10:90, and most preferably 0:100. An improved feelon hitting is obtainable by adjusting the blending proportion of baseresin components.

The base resin consisting of components (a) and (b) can also be tailoredfor effective molding by adjusting the blend ratio of the randomcopolymer and the neutralized random copolymer as well as theabove-adjusted proportion of components (a) and (b). It is recommendedthat the random copolymer and the neutralized random copolymer beblended in a weight ratio between 0:100 and 60:40, preferably 0:100 and40:60, more preferably 0:100 and 20:80, and most preferably 0:100. Withtoo high a proportion of the random copolymer, the material may becomedifficult to mix and mold.

In the inventive mixture, component (c) is a fatty acid or fatty acidderivative having a molecular weight of 280 to 1,500 whose purpose is toenhance the flow characteristics of the mixture. It has a molecularweight which is much smaller than that of the base resin, and serves toincrease the melt viscosity of the mixture to an appropriate level.Also, because the fatty acid or fatty acid derivative has a high contentof acid groups or derivative moieties thereof, its addition to themixture precludes a substantial loss of rebound.

The molecular weight of fatty acid or fatty acid derivative (c) is atleast 280, preferably at least 300, more preferably at least 330, andmost preferably at least 360 and up to 1,500, preferably up to 1,000,more preferably up to 600, and most preferably up to 500. Too low amolecular weight fails to improve heat resistance whereas too high amolecular weight fails to improve flow.

The fatty acid or fatty acid derivative (c) may be an unsaturated fattyacid or derivative thereof having a double bond or triple bond in thealkyl group, or it may be a saturated fatty acid or derivative thereofin which all the bonds on the alkyl group are single bonds. It isrecommended that the number of carbon atoms on the molecule generally beat least 18, preferably at least 20, more preferably at least 22, andeven more preferably at least 24, but up to 80, preferably up to 60,more preferably up to 40, and even more preferably up to 30. Too fewcarbons may make it impossible to achieve the improved heat resistance,and may also set the acid group content so high as to cause the acidgroups to interact with acid groups present on the base resin,diminishing the flow-improving effects. On the other hand, too manycarbons increases the molecular weight, which may also lower theflow-improving effects.

Examples of fatty acids (c) include stearic acid, 12-hydroxystearicacid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidicacid and lignoceric acid. Of these, stearic acid, arachidic acid,behenic acid and lignoceric acid are preferred, with behenic acid beingmost preferred.

Fatty acid derivatives which may be used as component-(c) includemetallic soaps in which the proton on the acid group of the fatty acidhas been substituted with a metal ion. Metal ions that may be used insuch metallic soaps include Na⁺, Li⁺, Ca²⁺, Mg²⁺, Zn²⁺, Mn²⁺, Al³⁺,Ni²⁺, Fe²⁺, Fe³⁺, Cu²⁺, Sn²⁺, Pb²⁺ and Co²⁺. Of these, Ca²⁺, Mg²⁺ andZn²⁺ are especially preferred.

Examples of the fatty acid derivatives (c) include magnesium stearate,calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate,calcium arachidate, zinc arachidate, magnesium behenate, calciumbehenate, zinc behenate, magnesium lignocerate, calcium lignocerate andzinc lignocerate. Of these, magnesium stearate, calcium stearate, zincstearate, magnesium arachidate, calcium arachidate, zinc arachidate,magnesium behenate, calcium behenate, zinc behenate, magnesiumlignocerate, calcium lignocerate and zinc lignocerate are preferred.

In the practice of the invention, use may also be made of known metallicsoap-modified ionomer resins, including those described in U.S. Pat. No.5,312,857, U.S. Pat. No. 5,306,760 and WO 98/46671, as a combination ofthe base resin (consisting of components (a) and (b)) with component(c).

The golf ball material of the invention includes as essential component(d) a basic inorganic metal compound capable of neutralizing the acidgroups in the base resin and component (c). As already noted in thepreamble, heating and mixing only a metal soap-modified ionomer resinfree of component (d) (e.g., only a metallic soap-modified ionomer resinof the type described in the above-cited patents) results in formationof a large amount of fatty acid due to an exchange reaction between themetallic soap and unneutralized acid groups on the ionomer resin, asshown below. This causes molding defects because the fatty acid thusformed is thermal unstable and readily vaporizes during molding. Inaddition, the fatty acid thus formed settles on the surface of themolded part, substantially lowering the ability of a paint film toadhere thereto.

Here, (1) is an unneutralized acid group present on the ionomer resin,(2) is a metallic soap, (3) is a fatty acid, and X is a metal atom.

In order to resolve such problems, the present invention incorporates ascomponent (d) a basic inorganic metal compound which neutralizes theacid groups present in the base resin and in component (c), therebyyielding a golf ball material having outstandingly improved reboundproperties due to the synergistic cooperation of these components.Incorporating essential component (d) serves to neutralize the acidgroups in the base resin and in component (c) to an appropriate extent.These components, when blended together in an optimum proportion, actsynergistically to increase the thermal stability of the mixture, impartgood moldability and enhance rebound characteristics.

It is recommended that the basic inorganic metal compound (d) be highlyreactive with the base resin to form reaction by-products which are freeof organic acid so that the degree of neutralization of the mixture maybe increased without detracting from thermal stability.

Exemplary metal ions that can be used in the basic inorganic metalcompound (d) include Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, Al³⁺, Ni²⁺, Fe²⁺,Fe³⁺, Cu²⁺, Mn²⁺, Sn²⁺, Pb²⁺ and Co²⁺. Examples of the basic inorganicmetal compound include well-known basic inorganic fillers containingthese metal ions, such as magnesium oxide, magnesium hydroxide,magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate,calcium oxide, calcium hydroxide, lithium hydroxide and lithiumcarbonate. Of these, hydroxides and monoxides are preferred. Inter alia,calcium hydroxide and magnesium oxide, especially calcium hydroxide, arepreferred since they are more reactive with the base resin.

The inventive golf ball material comprising, as described above, a baseresin consisting of a specific proportion of components (a) and (b), inadmixture with specific proportions of component (c) and component (d)has improved thermal stability, flow and moldability, and endows amolded part with outstanding rebound characteristics. It is criticalthat the components (c) and (d) be compounded in relative proportionsper 100 parts by weight of the base resin consisting of components (a)and (b); at least 5 parts, preferably at least 10 parts, more preferablyat least 15 parts, and most preferably at least 18 parts by weight, butnot more than 80 parts, preferably not more than 40 parts, morepreferably not more than 25 parts by weight, and most preferably notmore than 22 parts by weight, of component (c); and at least 0.1 part,preferably at least 0.5 part, more preferably at least 1 part, and mostpreferably at least 2 parts by weight, but not more than 10 parts,preferably not more than 8 parts, more preferably not more than 6 parts,and most preferably not more than 5 parts by weight, of component (d).Too little component (c) lowers the melt viscosity, resulting ininferior processability, whereas too much detracts from the durability.Too little component (d) fails to improve thermal stability and rebound,whereas too much component (d) instead lowers the heat resistance of thematerial due to the excess of basic inorganic metal compound.

While the golf ball material of the invention is arrived at by mixingthe base resin with components (c) and (d), it is recommended that atleast 60 mol %, preferably at least 70 mol %, more preferably at least80 mol %, and most preferably at least 90 mol %, of the acid groups inthe mixture be neutralized. Such a high degree of neutralization makesit possible to more reliably suppress the exchange reaction whichbecomes a problem when only the above-described base resin and the fattyacid or fatty acid derivative are used as in the prior art, and thusprevents the formation of fatty acid. As a result, there can be obtaineda material having greatly increased thermal stability, good moldabilityand a much larger resilience than prior-art ionomer resins.

It is noted that the degree of neutralization refers to the degree ofneutralization of acid groups in the mixture of the base resin and thefatty acid or derivative thereof (c) rather than the degree ofneutralization of an ionomer resin itself which is used as theneutralized random copolymer in the base resin. When the inventivemixture is compared with an ionomer resin alone having an identicaldegree of neutralization, the mixture contains much more metal ions.Then the mixture, when molded, arrives at a higher density of ioniccrosslinks contributing to resilience improvement, with a molded partbeing endowed with greater resilience.

To more reliably achieve both a high degree of neutralization and smoothflow, it is recommended that the acid groups in the mixture beneutralized with transition metal ions and alkali metal and/or alkalineearth metal ions. Although transition metal ions have weaker ioniccohesion than alkali metal and alkaline earth metal ions, the combineduse of metal ions of different species to neutralize the acid groups inthe mixture can provide a substantial improvement in flow.

It is recommended that the molar ratio between the transition metal ionsand the alkali metal and/or alkaline earth metal ions fall in the rangeof from 10:90 to 90:10, preferably from 20:80 to 80:20, more preferablyfrom 30:70 to 70:30, and most preferably from 40:60 to 60:40. Too low amolar ratio of transition metal ions may fail to provide sufficientimprovement in flow, whereas too high a molar ratio may lowerresilience.

Illustrative, non-limiting examples of the metal ions include zinc ionsas the transition metal ions, and at least one type of ion selected fromamong sodium ions, lithium ions and magnesium ions as the alkali metalor alkaline earth metal ions.

Any known method may be used in obtaining a mixture in which the desiredproportion of the acid groups have been neutralized with transitionmetal ions and alkali metal or alkaline earth metal ions. For example,specific methods of neutralization with transition metal ions, and inparticular zinc ions, include the use of zinc soap as the fatty acidderivative, the inclusion of a zinc-neutralized random copolymer (e.g.,zinc-neutralized ionomer resin) as components (a) and (b) in the baseresin, and the use of a zinc compound, typically zinc oxide, as thebasic inorganic metal compound (d).

The golf ball material of the invention can be prepared for a particularapplication by incorporating in the above-described mixture of essentialcomponents whatever additives may be required. For example, where thematerial is to be used as a cover stock, the mixture may have addedthereto such additives as pigments, dispersants, antioxidants,ultraviolet absorbers and light stabilizers.

Where such additives are compounded, the addition amount is preferablyat least 0.1 part, more preferably at least 0.5 part, and even morepreferably at least 1 part by weight and up to 10 parts, more preferablyup to 6 parts, and even more preferably up to 4 parts by weight, per 100parts by weight of the essential components (base resin+(c)+(d))combined.

In the golf ball material of the invention, a thermoplastic elastomerother than the ionomer resins may be compounded with the above-mentionedessential components in order to improve the hitting feel. Illustrativeexamples of the non-ionomer thermoplastic elastomer include olefinelastomers, styrene elastomers, polyester elastomers, urethaneelastomers, and polyamide elastomers. Of these, olefin elastomers andpolyester elastomers are preferred. The amount of the non-ionomerthermoplastic elastomer, when added, is preferably at least 1 part, morepreferably at least 2 parts, even more preferably at least 3 parts, mostpreferably at least 4 parts by weight and up to 100 parts, morepreferably up to 60 parts, even more preferably up to 40 parts, mostpreferably up to 20 parts by weight, per 100 parts by weight of theessential components (base resin+(c)+(d)) combined.

The golf ball material of the invention can be arrived at by furnishinga mixture of the above-described essential components and optionalcomponents. For instance, necessary ingredients are heated and mixed ata heating temperature of 150 to 250° C. and in an internal mixer such asa kneading-type twin-screw extruder, a Banbury mixer or a kneader. Anydesired method may be used to incorporate various additives togetherwith the essential components in the golf ball material of theinvention. For example, the additives may be blended with the essentialcomponents, and heating and mixing of all the ingredients carried out atthe same time. Alternatively, the essential components may be pre-heatedand pre-mixed, following which the optional additives may be added andthe overall composition subjected to additional heating and mixing.

The golf ball material of the invention is preferably adjusted to anappropriate melt flow rate to provide an adequate flow for injectionmolding, that is, to improve moldability. It is recommended that themelt flow rate (MFR) of the material, as measured in accordance withJIS-K7210 at a temperature of 190° C. and under a load of 21.18 N (2.16kgf), be at least 0.5 dg/min, preferably at least 1 dg/min, morepreferably at least 1.5 dg/min, and most preferably at least 2.0 dg/minand not more than 20 dg/min, preferably not more than 10 dg/min, morepreferably not more than 5 dg/min and most preferably not more than 3dg/min. If the mixture has too low or too high a melt flow rate, theprocessability may decrease markedly.

The golf ball material of the invention is preferably optimized in therelative absorbance in infrared absorption-spectroscopy, representingthe ratio of absorbance at the absorption peak attributable tocarboxylate anion stretching vibrations normally detected at 1530 to1630 cm⁻¹ to the absorbance at the absorption peak attributable tocarbonyl stretching vibrations normally detected at 1690 to 1710 cm⁻¹.For the sake of clarity, this ratio may be expressed as: (absorbance ofabsorption peak for carboxylate stretching vibrations)/(absorbance ofabsorption peak for carbonyl stretching vibrations).

Here, “carboxylate stretching vibrations” refers to vibrations bycarboxyl groups from which the proton has dissociated (metalion-neutralized carboxyl groups), whereas “carbonyl stretchingvibrations” refers to vibrations by undissociated carboxyl groups. Theratio in these respective peak intensities depends on the degree ofneutralization. For commonly used ionomer resins having a degree ofneutralization of about 50 mol %, the ratio between these peakabsorbances is about 1:1.

To improve the thermal stability, flow, moldability and rebound of thegolf ball material, it is recommended that the material have acarboxylate stretching vibration peak absorbance which is at least 1.3times, preferably at least 1.5 times, and more preferably at least 2times, the carbonyl stretching vibration peak absorbance. The absence ofa carbonyl stretching vibration peak altogether is especially preferred.

The thermal stability of the inventive golf ball material can bemeasured by thermogravimetry. It is recommended that, inthermogravimetric analysis, the mixture have a weight loss at 250° C.,based on the weight of the mixture at 25° C., of not more than 2% byweight, preferably not more than 1.5% by weight, and most preferably notmore than 1% by weight.

It is further recommended that the compounding of the golf ball materialbe adjusted so as to provide a molded part thereof with a Shore Dhardness of at least 30, preferably at least 40, more preferably atleast 45, even more preferably at least 50, but up to 60, preferably upto 58, more preferably up to 56, even more preferably up to 54. Too higha Shore D hardness may compromise the feel of a golf ball made of thematerial when hit whereas too low a Shore D hardness may lead to adecline of resilience.

The golf ball material may have any desired specific gravity although itis generally advisable for the specific gravity to be at least 0.9, morepreferably at least 0.92, even more preferably at least 0.94, but notmore than 1.2, more preferably not more than 1.1, even more preferablynot more than 1.05.

The golf ball of the invention has a molded part of the golf ballmaterial according to the invention as a constituent component. Thelayer or layers made of the golf ball material may constitute a portionor all of the golf ball. The inventive golf balls may be thread-woundballs, including those in which the cover has a single-layer or amultiple-layer construction, one-piece balls, two-piece balls,three-piece balls, or multi-piece balls having a cover composed of threeor more layers. The type of golf ball is not critical as long as theball has a molded part of the inventive golf ball material as aconstituent component.

The inventive golf balls may be manufactured by preparing variousmixtures for making one-piece balls, the solid centers of thread-woundgolf balls, the solid cores of solid golf balls, or cover stock (for atleast one layer in cores and covers composed of two or more layers) inaccordance with the above-described golf ball material formulation ofthe invention, then using the mixture in accordance with a golf ballmanufacturing method known to the art.

When the cover of a golf ball is made of the golf ball materialaccording to the present invention, the core may be either athread-wound core or a solid core and may be produced by a conventionalmethod.

For example, a solid core may be produced by preparing a rubbercomposition composed of 100 parts by weight of cis-1,4-polybutadiene;from 10 to 60 parts by weight of one or more vulcanizing or crosslinkingagents selected from among α,β-monoethylenically unsaturated carboxylicacids (e.g., acrylic acid, methacrylic acid) or metal ion-neutralizedcompounds thereof and functional monomers (e.g., trimethylolpropanemethacrylate); from 5 to 30 parts by weight of a filler such as zincoxide or barium sulfate; from 0.5 to 5 parts by weight of a peroxidesuch as dicumyl peroxide; and, if necessary, from 0.1 to 1 part byweight of an antioxidant. The resulting rubber composition can be formedinto a solid spherical core by press vulcanization to effectcrosslinkage, followed by compression under heating at 140 to 170° C.for a period of 10 to 40 minutes.

Production of a thread-wound golf ball core may be carried out usingeither a liquid or a solid center. In the case of a liquid center, ahollow spherical center envelope may be formed from the above-describedrubber composition, for example, and a liquid filled into this envelopeby a well-known method. If a solid center is used instead, the solidcenter may be produced by the solid core production method describedabove. Thereafter, rubber thread is wound in a stretched state about thecenter to form the core. Use may be made of rubber thread produced by aconventional method. For example, rubber thread is prepared bycompounding natural rubber or synthetic rubber such as polyisoprene withvarious additives (e.g., antioxidants, vulcanization accelerators andsulfur) to form a rubber composition, which is molded and vulcanized.

The golf balls using the various types of cores described above andfalling within the scope of the invention can be produced by forming thecover from the inventive golf ball material. In one such method, asingle-layer or multi-layer core prefabricated according to the type ofball to be manufactured is placed in a mold, and the inventive materialis heated, mixed and melted, then injection-molded over the core. Inthis case, the golf ball manufacturing operation can be carried outunder conditions which assure that the material maintain excellentthermal stability, flow and moldability. The resulting golf ball has ahigh rebound.

The method used to produce the cover is not limited to the methoddescribed above. In an alternative method which can be used herein, apair of hemispherical cups is molded from the inventive golf ballmaterial, following which the cups are placed over a core and moldedunder heat (120 to 170° C.) and pressure for 1 to 5 minutes. Noparticular limitation is imposed on the thickness of the cover made ofthe inventive material, although the cover is generally formed to athickness of at least 0.5 mm, preferably at least 0.9 mm, morepreferably at least 1.1 mm, but up to 3 mm, preferably up to 2.5 mm,more preferably up to 2.3 mm. The cover in the golf balls of theinvention is not limited to one layer, and may instead have a multilayerconstruction of two or more layers. If the cover has a multilayerconstruction, the golf ball material of the invention may be used eitherat the interior of the multilayer construction or as the outermost layerof the cover. In the case of a single-layer cover (for two-piece golfball), it is highly advantageous for the inventive material to serve asthe cover material. If the ball is a multi-piece golf ball having acover of two or more layers, the inventive material is most preferablyused as a layer of the cover other than the outermost layer—that is, asan inner layer of the cover. In this embodiment, the cover outermostlayer is preferably formed of an ionomer resin.

The golf ball may have a plurality of dimples formed on its surface, andthe cover may be administered various treatment such as surfacepreparation, stamping and painting. In particular, a golf ball covermade of the inventive material ensures ease of work involved inadministering such surface treatment.

The golf ball of the invention may be a golf ball in which the inventivegolf ball material is used other than as the cover stock describedabove. For example, it may be a golf ball arrived at by using theinventive material as a one-piece golf ball material or as a corematerial. In this case, production may be carried out using well-knownmethods.

In the golf balls manufactured as described above, the diameter, weight,hardness and other parameters of the cover, solid or liquid center,solid core or thread-wound core, and one-piece golf balls, while notsubject to any particular limitations, may be adjusted as appropriate,insofar as the objects of the invention are attainable.

The golf ball of the invention may be manufactured for use intournaments by giving it a diameter and weight which conform with theRules of Golf. That is, the ball may be produced to a diameter of notless than 42.67 mm and a weight of not greater than 45.93 g.

EXAMPLE

Examples of the invention and comparative examples are given below byway of illustration, and are not intended to limit the invention.

Examples 1-7 and Comparative Examples 1-7

Using a core material composed primarily of cis-1,4-polybutadiene, asolid core was produced having a diameter of 38.6 mm, a weight of 35.1g, and a deflection of 3.1 mm under a load of 980 N (100 kg).

Cover materials of the compositions shown in Tables 1 and 2 were mixedat 230° C. with a kneading-type twin-screw extruder and prepared in theform of pellets. In each of the examples, the cover material wasinjected into a mold in which the solid core prepared above had beenplaced, giving a two-piece solid golf ball having a diameter of 42.8 mmand a cover thickness of 2.1 mm.

Example 8 and Comparative Examples 8 and 9

Using a core material composed primarily of cis-1,4-polybutadiene, asolid core was produced having a diameter of 36.4 mm, a weight of 30.9g, and a deflection of 3.9 mm under a load of 980 N (100 kg).

In Example 8, the cover material described above in Example 1 wasinjection-molded over the core so as to form a cover inner layer havinga thickness of 1.7 mm. Similarly, in Comparative Examples 8 and 9, therespective cover materials described in Comparative Examples 1 and 2were injection-molded over the core so as to form a cover inner layerhaving a thickness of 1.7 mm. Next, in each of the three examples, theouter cover material shown in Table 3 was injection-molded over thecover inner layer, thereby giving a three-piece solid golf ball having adiameter of 42.8 mm.

The following characteristics were measured or evaluated for the golfballs obtained in each of the above examples. The results are also shownin Tables 1 to 3.

Ball Hardness:

Measured as the deflection (in millimeters) of the ball under an appliedload of 980 N (100 kg).

Initial Velocity:

Measured using the same type of initial velocity instrument as thatapproved by the United States Golf Association (USGA), and in accordancewith USGA rules.

Relative Absorbance of Carboxylate Absorption Peak:

A transmission method was used to measure the infrared absorption of thesamples. In the infrared absorption spectrum for a sample prepared tosuch a thickness as to make the peak transmittance associated withhydrocarbon chains observed near 2900 cm⁻¹ about 90%, the absorptionpeak due to carbonyl stretching vibrations (1690 to 1710 cm⁻¹) wasassigned an absorbance value of 1 and the ratio thereto of theabsorption peak due to carboxylate anion strength vibrations (1530 to1630 cm⁻¹) was computed as the relative absorbance.

Percent Weight Loss:

Prior to measurement, samples were dried in a dry hopper at 50° C. for24 hours for eliminating the influence of moisture. Thermogravimetricanalysis was carried out on approximately 5 mg samples by raising thetemperature from 25° C. to 300° C. in a nitrogen atmosphere (flow rate,100 ml/min) at a rate of 10° C./min, then calculating the percent lossin the sample weight at 250° C. relative to the sample weight at 25° C.

Melt Flow Rate:

The melt flow rate of the material was measured in accordance withJIS-K7210 at a temperature of 190° C. and under a load of 21.18 N (2.16kgf).

Extrudability:

Each of the cover materials was worked at 200° C. in an intermeshingco-rotating type twin-screw extruder (screw diameter, 32 mm; main motoroutput, 7.5 kW) such as is commonly used for mixing materials, whereuponit was rated for processability as follows.

Good: Extrudable

Poor: Cannot be extruded due to excess loading

Trade names and materials mentioned in the tables are described below.

-   Nucrel 1560: Ethylene-methacrylic acid copolymer made by    DuPont-Mitsui Polychemicals Co., Ltd. Acid content, 15 wtt.-   Himilan 1605: Sodium neutralized ethylene-methacrylic acid copolymer    made by DuPont-Mitsui Polychemicals Co., Ltd.-   Himilan 1706: Zinc neutralized ethylene-methacrylic acid copolymer    made by DuPont-Mitsui Polychemicals Co., Ltd.-   Nucrel AN4318: Ethylene-methacrylic acid-acrylate copolymer made by    DuPont-Mitsui Polychemicals Co., Ltd. Ester content, 17 wt %.-   Surlyn 8320: Sodium neutralized ethylene-methacrylic acid-acrylate    copolymer made by DuPont.-   Surlyn 9320: Zinc neutralized ethylene-methacrylic acid-acrylate    copolymer made by DuPont.-   Himilan 1856: Sodium neutralized ethylene-methacrylic acid-acrylate    copolymer made by DuPont-Mitsui Polychemicals Co., Ltd.-   Himilan 1855: Zinc neutralized ethylene-methacrylic acid-acrylate    copolymer made by DuPont-Mitsui Polychemicals Co., Ltd.-   Surlyn 6320: Magnesium neutralized ethylene-methacrylic    acid-acrylate copolymer made by DuPont.-   Himilan 1601: Sodium neutralized ethylene-methacrylic acid copolymer    made by DuPont-Mitsui Polychemicals Co., Ltd.-   Himilan 1557: Zinc neutralized ethylene-methacrylic acid copolymer    made by DuPont-Mitsui Polychemicals Co., Ltd.-   Behenic acid: Produced by NOF Corporation under the trade name    NAA-222S.-   Calcium hydroxide: Produced by Shiraishi Industry Co., Ltd. under    the trade name CLS-B.

TABLE 1 Example 1 2 3 4 5 6 7 Composition Component Nucrel 1560 20.0(pbw) (a) Himilan 1605 10.0 Himilan 1706 10.0 Component Nucrel AN431820.0 (b) Surlyn 8320 50.0 40.0 40.0 40.0 40.0 50.0 Surlyn 9320 50.0 40.040.0 40.0 40.0 50.0 Himilan 1856 50.0 Himilan 1855 50.0 Surlyn 6320 20.0Component Behenic acid 20.0 20.0 20.0 20.0 20.0 20.0 (c) Calciumstearate 20.0 Component Calcium hydroxide 2.5 2.5 2.5 3.0 3.0 3.5 0.5(d) Titanium dioxide 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Resin Extrudabilitygood good good good good good good properties Melt flow rate (dg/min)2.0 2.0 2.0 2.0 1.8 1.8 1.5 Weight loss (wt %) 0.7 0.6 1.0 0.6 0.6 0.61.5 Relative absorbance of 1.8 1.9 1.8 1.9 2.0 1.9 2.0 carboxylate peakCover hardness (Shore D) 50 56 50 50 53 53 50 Specific gravity 0.97 0.970.97 0.97 0.97 0.97 0.97 Ball Weight (g) 45.2 45.2 45.2 45.2 45.2 45.245.2 properties Hardness (mm) 2.82 2.76 2.82 2.82 2.80 2.79 2.82 Initialvelocity (m/s) 77.2 77.6 77.2 77.1 77.5 77.4 77.2

TABLE 2 Comparative Example 1 2 3 4 5 6 7 Composition Component Nucrel1560 (pbw) (a) Himilan 1605 10.0 10.0 10.0 50.0 Himilan 1706 10.0 10.010.0 Component Nucrel AN4318 (b) Surlyn 8320 50.0 50.0 50.0 40.0 40.040.0 Surlyn 9320 50.0 50.0 50.0 40.0 40.0 40.0 50.0 Himilan 1856 Himilan1855 Surlyn 6320 Component Behenic acid (c) Calcium stearate 20.0 20.0Component Calcium hydroxide 2.5 3.0 (d) Titanium dioxide 2.0 2.0 2.0 2.02.0 2.0 2.0 Resin Extrudability good good poor good good poor goodproperties Melt flow rate (dg/min) 1.0 1.3 — 1.2 1.5 — 1.6 Weight loss(wt %) 0.5 2.8 — 0.5 2.8 — 0.5 Relative absorbance of 1.1 1.7 — 1.0 1.6— 0.6 carboxylate peak Cover hardness (Shore D) 45 48 — 50 53 — 50Specific gravity 0.97 0.97 — 0.97 0.97 — 0.97 Ball Weight (g) 45.2 45.2— 45.2 45.2 — 45.2 properties Hardness (mm) 2.87 2.84 — 2.83 2.79 — 2.83Initial velocity (m/s) 76.6 76.8 — 76.9 77.0 — 76.9

TABLE 3 Comparative Example Example 8 8 9 Cover inner CompositionComponent Surlyn 8320 50.0 50.0 50.0 layer (pbw) (b) Surlyn 9320 50.050.0 50.0 Component Behenic acid 20.0 (c) Calcium stearate 20.0Component Calcium hydroxide 2.5 (d) Titanium dioxide 2.0 2.0 2.0 Coverouter Composition Himilan 1601 50.0 50.0 50.0 layer (pbw) Himilan 155750.0 50.0 50.0 Titanium dioxide 2.0 2.0 2.0 Cover hardness (Shore D) 6060 60 Specific gravity 0.97 0.97 0.97 Ball Weight (g) 45.2 45.2 45.2properties Hardness (mm) 2.85 2.89 2.87 Initial velocity (m/s) 76.7 76.276.5

The cover stocks of Examples 1 to 7 had superior resilience to theionomer resin blend cover stocks of Comparative Examples 1, 4 and 7;superior resilience and thermal stability to the metal soap-modifiedcover stocks of Comparative Examples 2 and 5; and superior moldabilityto the highly neutralized ionomer resin cover stocks of ComparativeExamples 3 and 6.

The three-piece solid golf ball of Example 8 using the cover stock ofExample 1 as the intermediate layer had superior rebound to thethree-piece solid golf balls of Comparative Examples 8 and 9 using thecover stocks of Comparative Examples 1 and 2 as the intermediate layer.

The golf-ball material of the Invention has good thermal stability, flowcharacteristics and moldability, and can be molded into a partexhibiting excellent rebound, ensuring the manufacture of golf ballswith high performance. The golf balls of the invention having a moldedpart of the inventive golf ball material as a constituent component canbe manufactured easily and efficiently, and have excellent rebound.

Japanese Patent Application No. 2000-379247 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A golf ball comprising a solid core and a cover of two or more layersaround the core, wherein the solid core is composed primarily ofcis-1,4-polybutadiene and a cover inner layer other than a coveroutermost layer is formed of a material comprising a mixture which iscomposed of: 100 parts by weight of a base resin having (a) anolefin-unsaturated carboxylic acid binary random copolymer or a metalion-neutralized olefin-unsaturated carboxylic acid binary randomcopolymer or both, blended with (b) an olefin-unsaturated carboxylicacid-unsaturated carboxylate ternary random copolymer or a metalion-neutralized olefin-unsaturated carboxylic acid-unsaturatedcarboxylate ternary random copolymer or both, in a weight ratio of 0:100to 30:70, (c) 5 to 80 parts by weight of a fatty acid or fatty acidderivative or both, having a molecular weight of 280 to 1,500; and (d)0.1 to 10 parts by weight of a basic inorganic metal compound capable ofneutralizing acidic groups left unneutralized in the base resin andcomponent (c), and the cover outermost layer is formed primarily of anionomer resin.
 2. The golf ball of claim 1, wherein the mixture whenmolded has a Shore D hardness of 30 to
 60. 3. The golf ball of claim 1,wherein the mixture has a melt index of 0.5 to 20 dg/min.
 4. The golfball of claim 1, wherein the metal ions for neutralizing acid groups inthe metal ion-neutralized olefin-unsaturated carboxylic acid binaryrandom copolymer (a) or metal ion-neutralized olefin-unsaturatedcarboxylic acid-unsaturated carboxylate ternary random copolymer (b) areselected from the group consisting of Na⁺, K⁺, Zn²⁺, Cu²⁺, Mg²⁺, Ca²⁺,Co²⁺, Ni²⁺, and Pb²⁺.
 5. The golf ball of claim 1, wherein the metalion-neutralize random copolymer in the base resin comprises a zincion-neutralized ionomer resin.
 6. The golf ball of claim 1, wherein theneutralized random copolymers are prepared by neutralizing a randomcopolymer with one or more metal ions wherein the random copolymers areneutralized using a compound selected from the group consisting offormates, acetates, nitrates, carbonates, hydrogencarbonates, oxides,hydroxides and alkoxides of the metal ions.
 7. The golf ball of claim 1,wherein the total content of random copolymers and the total content ofmetal ion-neutralized random copolymers in the base resin are in aweight ratio of 0:100 to 60:40.
 8. The golf ball of claim 1, wherein thefatty acid (c) is one or more compounds selected from the groupconsisting of stearic acid, 12-hydroxystearic acid, behenic acid, oleicacid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid.9. The golf ball of claim 1, wherein the fatty acid derivative (c) isone or more compounds selected from the group consisting of magnesiumstearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate,calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesiumarachidate, calcium arachidate, zinc arachidate, magnesium behenate,calcium behenate, zinc behenate, magnesium lignocerate, calciumlignocerate and zinc lignocerate.
 10. The golf ball of claim 1, whereinthe metal ion that can be used in the basic inorganic metal compound (d)is selected from the group consisting of Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺,A³⁺, Ni²⁺, Fe⁺, Fe⁺, Cu⁺, Mn²⁺, Sn²⁺, Pb²⁺ and Co²⁺.
 11. The golf ballof claim 1, wherein the basic inorganic metal compound (d) is one ormore compounds selected from the group consisting of magnesium oxide,magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide,sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxideand lithium carbonate.
 12. The golf ball of claim 1, wherein the acidgroups in the mixture are neutralized with transition metal ions andalkali metal and/or alkaline earth metal ions.
 13. The golf ball ofclaim 12, wherein the molar ratio between the transition metal ions andthe alkali metal and/or alkaline earth metal ions is in the range offrom 10:90 to 90:10.
 14. The golf ball of claim 12, wherein thetransition metal ions include zinc ions and at least one type of metalion in the alkali metal or alkaline earth metal ions is selected fromthe group consisting of sodium ions, lithium ions and magnesium ions.15. The golf ball of claim 1, wherein the solid core is produced bypreparing a rubber composition composed of 100 parts by weight ofcis-1,4-polybutadiene, from 10 to 60 parts by weight of one or moreα,β-monoethylenically unsaturated carboxylic acids or metalion-neutralized compounds thereof, from 5 to 30 parts by weight of afiller, and from 0.5 to 5 parts by weight of a peroxide.